lambda

sensor:
  - platform: wifi_signal
    name: "Sonoff Basic WiFi Signal"
    update_interval: 60s
    filters:
    - lambda: return abs( (id(sensor.sensor01_temperature).state+id(sensor.sensor01_temperature).state)/2.0 - (id(sensor.sensor03_temperature).state+id(sensor.sensor04_temperature).state)/2.0);